Field of the Invention
The invention relates generally to nuclear reactors and more particularly to control rod drive mechanisms that raise and lower control rods used in the nuclear reactor.
In commercial nuclear reactors, heat, from which steam and ultimately electricity are generated, is produced by fissioning of a fissile material such as enriched uranium. This fissile material or nuclear fuel, is typically contained within a nuclear core made up of a number of fuel rods supported in a plurality of nuclear fuel assemblies, arranged in a spaced parallel array.
Movable control rods are dispersed throughout the core to control the fission process. The control rods generally comprise a plurality of elongated rods containing neutron absorbing materials which fit in longitudinal openings defined in the fuel assemblies and among the fuel rods by guide thimbles of the fuel assemblies. The guide thimbles guide the control rods during their movement into and out of the core. Inserting a control rod into the core adds more neutron absorber material and decreases the nuclear reaction; conversely, withdrawing a control rod removed neutron absorber material and increases the nuclear reaction and thereby the power output of the core. The nuclear reactor core and the control rods are positioned within and supported by a reactor vessel through which a reactor coolant flows.
The control rods are supported in cluster assemblies that can be moved into and from the nuclear core by control rod drive mechanisms which, in turn, are mounted by an upper internal arrangement located within the nuclear reactor vessel above the nuclear core. Typically, a reactor vessel is pressurized to a relatively high internal pressure. The control rod drive mechanisms operate within the same pressure environment that exists within the reactor vessel. The control rod drive mechanisms are housed within shaft housing which are tubular extensions of the reactor pressure vessel.
One of the more commonly used type of control rod drive mechanisms is referred to as a "magnetic jack". With this type of drive mechanism, the control rods are jacked into and from the nuclear core in a series of motions, each involving moving the control rod a discrete incremental distance or "step". Such movement is commonly referred to as stepping of the control rods. This magnetic jack type of mechanism is illustrated and described in U.S. Pat. No. 3,158,766 to Frisch and U.S. Pat. No. 3,992,225 to DeWesse which are assigned to the assignee of the present invention.
This magnetic jack type of control rod drive mechanism includes three electromagnetic coils and a plurality of armature discs which are operated to raise and lower a drive rod shaft and thereby the control rod cluster assembly. The three coils are mounted about and outside of the shaft housing. Two of the coils actuate respective shaft movable and stationary gripper discs contained within the shaft housing. The third coil actuates a lift disc. Actuation of the movable and stationary discs, in turn, operates sets of circumferentially spaced latches which grip the drive rod shaft having multiple axially-spaced circumferential grooves. The stationary gripper latches are actuated to hold the drive rod shaft in a desired axial position. The movable gripper latches are actuated to raise and lower the drive rod shaft. Each jacking or stepping movement of the control rod drive mechanism moves the drive rod shaft about 5/8 inch (1.58 cm) The jacking or stepping movement is accomplished by the operation of the three sets of axially spaced electromagnetic coils to actuate the corresponding stationary, movable and lift discs so as to alternately and sequentially grip, move and release the control rod drive shaft of the respective mechanism.
In the known magnetic jack type of control rod drive mechanisms, the three coils which are mounted about and outside of the shaft housing were spool type coils in which a spool of non-magnetic material was wound with circumferential wrappings of wire to form a donut shaped coil around the spool and the pressure housing from the shaft.
In the known design, the spool type coils could not be fitted directly against the outside wall surface of the shaft housing because of flanges and other appurtenances on the housing. As a result the spool type coils had to be larger than the outside diameter of the shaft housing and positioned away from the outside wall surface of the shaft housing. This arrangement also required the use of magnetic yoke and magnetic spacer rings to magnetically couple the coil to the magnetic discs contained within the shaft housing.
Such an arrangement required coils that were larger than necessary and the magnetic yokes and spacer rings added to the cost and complexity of the mechanism.
Another limitation to the known spool type coils was that they provided a symmetric magnetic field within the shaft housing which did not provide flexibility in operating the control rod shafts in response to any unbalanced loads on the shafts.